As you make backup for yourself, every program also wants to be in the safe side and fill all positions in the main match. Every program knows at what number and quality usually matches and just includes few extra backups. If a program needs four and usually matches every year by number 10 maximum, what is the point to interview 80 and ranks 50 for example? Because of that, candidates usually go to one of their top three unless there is a major problem in their application.